Optical fibers are used for form precisely defined or fixed optical paths for the propagation of coherent light in a number of measurement instruments, to develop signals indicative of a quantity which it is desired to measure, such as rotation or acceleration. The variation in some characteristic influenced by the physical quantity will depend in a direct manner on the length of the fiber, the number of turns in a coil, or the like. The construction of a sensitive instrument therefore commonly utilizes a fiber wound in a multi-turn coil. Various design constraints, such as limitations on permissible bending stresses in the fiber, light attenuation and the like, result in coil constructions which may consist of a fiber tens or many hundreds of meters long, wound into a coil of perhaps five to ten centimeters diameter.
In the prior art, such coils are made by winding many turns of an optical fiber about a spindle or other coilform, essentially forming a fiber spool. A principal design constraint in this type of construction is that the coilform must not introduce stresses into the fiber, since stresses can cause signal attenuation, polarization loss, or other deleterious effects.
When a twenty meter length of fiber is cooled to the temperature of liquid nitrogen, it shrinks by several millimeters. In the prior art constructions, if this shrinkage is not accommodated by corresponding changes of coilform dimensions, great stresses can build up in the fiber, leading to degradation of instrument parameters. Consequently, coilforms have been designed of materials with thermal constants selected to minimize differential thermal expansion between the coilform and the coiled fiber. Among the materials which have been used for coilforms are titanium, ceramics, aluminum, polycarbonate plastics and exotic carbon composites. An adhesive or potting material is also generally used in the completed coil assembly, and such adhesives or potting materials must also be selected to minimize the introduction of unwanted thermal effects.
To optimize the coil design for extreme temperature changes, however, is not a simple problem. The fibers themselves do not constitute a homogeneous structure, but utilize extremely dissimilar materials for the central light path component, i.e., the core and cladding which are generally formed of fused silica, and for the outer protective jacket, which is generally formed of one or more polymer layers. The thermal expansion properties of the fiber are thus those of a multi-component mechanical system, rather than of a bulk material. In addition, operation at cryogenic temperatures causes an increase of bending modulus in most fiber coatings, potting materials and adhesives, and this further increases stain due to microbending, introducing a concomitant degradation of fiber characteristics.
Considerable efforts have been made to develop thermal models for determining appropriate materials and structures for fiber/coilform assemblies to be able to operate predictably in the temperature range of -200.degree. C. to +100.degree. C. Accordingly, there is a need for a fiber coil with improved temperature dependent characteristics.